High volume and high flow rate pump casing designs have traditionally required several compromises. While larger casings can provide greater pump efficiencies, smaller casings are often used to reduce costs. Additionally, single-piece pump casings have often included cast internal portions which are very difficult to access. These pump casings have been shaped to balance competing considerations of casting ease, cost minimization, size constraints and flow efficiency. In high volume and high flow rate applications such as sea water reverse osmosis (SWRO) applications, increasing a few percentage points in efficiency can drastically decrease energy costs.